Siloxane polymers or copolymers having a high refractive index have been increasingly used for a variety of optical applications including, for example, in contact lenses, intraocular lenses, etc. Such polymers are also finding their way into other optical applications requiring high transmission and high refractive index including but not limited to, solid state lighting (light emitting diodes, organic light emitting diodes, laser diodes), waveguides (both planar and “fiber” geometries), optical computing, optical storage media, antireflection coatings, conformal coatings, optical lenses, microlenses, automobile topcoats, paint formulations, hair care products, gradient refractive index optical components, dynamic gradient refractive index components, etc.
Depending on the application, the polymers and products formed from such polymers may need to exhibit a wide range of properties including sufficient structural integrity, strength, elasticity and elongation, index of refraction, etc. In some applications, the polymers must exhibit these properties when formed into a thin film. For example, in intraocular lenses, the lens must be thin and pliable for insertion through a small incision in intraocular lens applications, be able to regain its original shape after incision, and have sufficient structural integrity and strength to retain such shape under normal use conditions.
Introduction of aromatic groups is now a general approach to increase the refractive index of siloxane polymers, and conventional co-polymers for high refractive index applications consist of dimethylsiloxane-phenylmethylsiloxane co-polymers or dimethylsiloxane-diphenylsiloxane co-polymers as described in, for example, U.S. Pat. Nos. 3,996,189; 5,147,396; 5,444,106; and 5,236,970, JP10305092, EP 0335312, WO 93/21245, and WO 95/17460. At a phenyl content of approximately 15 mole %, a polydimethyl siloxane/methylphenyl siloxane co-polymer has a refractive index of 1.462, (Eur. Polymer J. 1998, 34, 1727-1733).
Despite the positive effect on the refractive index, the introduction of refractive index modifying groups, such as phenyl-groups, in polysiloxanes is known to result in several disadvantages. Materials formed from siloxanes containing phenyl groups can have reduced flexibility, poor mechanical strength and elasticity, and they can also be hard and brittle. Further, materials with phenyl content greater than 40 mol % are not easily processed and tend to exhibit poor mechanical strength. This limits the refractive index that can be achieved to about 1.54.
One remedy to the problem of vulnerability to cracking is to reinforce the optical structure and improve its mechanical properties by combining the polymer with a solid filler material. Mostly, finely powdered silica is used as a filler material for this purpose. This filler material has a refractive index of 1.46. Since differences in the refractive index of the filler material and the polymer are not allowable in an optical lens, the maximum refractive index of a lens containing such filler material is ultimately 1.46.
In addition to issues with mechanical properties, the incorporation of phenyl into the silicones also makes resulting polymers more vulnerable under thermal and UV-exposure conditions. This results in yellowing of the optical material and transmission losses such that the transmission level is below a tolerable level and can lead to mechanical failure of a device in the optical components. There is a need for alternative means of altering the refractive index of siloxanes.